This invention relates generally to pipe joints, and more specifically to methods and apparatus for joining similar pieces of plastic pipe to each other where the pipe is made of material that has a “shape memory” that permits slight temporary deformation of the pipe.
Plastic pipe, including corrugated drainpipe, is used extensively in the United States and foreign countries. In many applications, similarly sized and shaped lengths of pipe are fabricated and transported to installation sites, where they are assembled with each other to form a desired extended flowpath for certain fluids. The size and shape of the pipes, as well as the fluids handled and the installation sites, can vary widely. By way of example, corrugated polyethylene drainpipe is commonly 12″ diameter and 20′ long. Joining the pipe pieces to each other can be an important, time-consuming, and costly process, especially for large-diameter, long pipes where leakage at the joint must be controlled or minimized.
In the United States alone, well over 1 billion pounds of plastic corrugated drainpipe are manufactured and installed per year, and it is gaining market share rapidly in relationship to competing products (such as those constructed of concrete, steel, or clay). The reasons for this market shift toward plastic pipe include many economic and other factors. By way of example, a 12″ diameter corrugated drainpipe made of polyethylene weighs only approximately 3.2 pounds/foot, as compared to much heavier concrete or clay pipe (about 79 lbs./ft.) and corrugated steel (10.5 lbs./ft.). The lighter weight provided by plastic materials has corresponding benefits in reducing freight costs, improving the rate of installation, and improving safety when handling and storing the pipe. Again, by way of example, polyethylene corrugated drain pipe has existed for over 20 years and appears to have a secure future. Other plastic pipes have similar current and likely continuing high usage rates.
As indicated above, in certain applications and industries (such as the polyethylene corrugated drainpipe industry), joints between pieces of pipe need to be leak proof and pressure resistant. This pressure resistance to avoid leaking can be critical, for example, in sewage applications, under roadways, and in other applications where watertightness is required. Current joints are unsatisfactory/unreliable for such applications, are expensive, or both.
Integral bell and spigot systems are very common in the drainpipe industry. Some of the most watertight couplings (although they are not sufficiently watertight) currently used to join these large drainpipes are referred to as Bell/Bell couplings. These are generally cylindrical sleeves into which the ends of the pipe are inserted. The couplings are not integrally formed with the pipe pieces, but instead are independently formed and constructed. The Bell/Bell name arises from their typical use of end-to-end or “back to back” “bells” (when viewed in cross-section), each drainpipe end being joined to the other by being inserted into one of the opposing bells. These couplings commonly use “O” ring systems to assist in sealing, with the O-ring positioned around the valley between the first and second rung of the ends of the drainpipe.
A commonly used prior art apparatus and method of “bell/bell couplings” is described in U.S. Pat. No. 5,326,138, which is incorporated herein by reference. Among other things, the approach shown in that patent is costly to manufacture and install, and can be unwieldy to handle.
In addition, tolerance variations commonly found in polyethylene drainpipe are approximately +/−8%. These dimensional variations contribute to the unreliability of current “pressure resisting” couplings and bell and spigot designs. The present methods are so unreliable that a common term used to describe the products is “soil tight” rather than “watertight”. “Soil tight” joints usually leak between 0.5 and 4.5 pounds of pressure. In contrast, “watertight” fittings are supposed to meet 10.8 pounds of pressure testing requirements of ASTM D3212. Because those ASTM requirements are for controlled testing conditions and because present coupling technology is so “leaky”, if a contract requires post-installation pressure testing of the installed pipe, few if any companies will even bid to use plastic corrugated drainpipe on the project, and instead will only bid to use only clay or some other type of pipe.
In some smaller diameter applications, tubing pieces are joined by compression exerted between the respective pipe walls. Although this approach creates a relatively highly waterproof connection, the two pipe ends typically have to be formed in different diameters. This permits the smaller pipe end to be inserted into the larger pipe end, so that the pieces can be forced together into a compression fit/joint.
Similar use of compression joints for differently sized pipes/openings include irrigation systems wherein a ¼″ polyethylene tube is pushed through a ⅛″ hole in the side wall of 1″ diameter polyethylene tube. Such joints are held together by shape memory means, and can withstand 25 psi for as long as 30 years (based on certain installations in the Coachella Valley of California, even though the joints are on the earth's surface and therefore have been exposed to the sun and elements throughout most or all of that time. Neither this approach or the one discussed in the previous paragraph provide any way to join the ends of lengths of similarly sized and shaped pipe pieces, but instead require more complicated manufacturing and installation (the pieces are not bi-directional, have to be drilled through the sidewall, etc.).
Other examples of coupling approaches include overlapping joint methods (such as shown in U.S. Pat. No. 4,636,272 to Riggs, and U.S. Pat. No. 4,204,897 to Bartell), and the present inventor's U.S. Pat. No. 6,090,233. Although the latter does join two similarly sized and shaped pieces of very flexible small tubing, the joint and necessary seal between the two pieces is accomplished by simultaneously applying heat and pressure to fuse the two pieces of tubing to each other, and applying that heat and pressure while the pieces are flattened. Such an approach is not usable in plastic pipe that has “shape memory”(such as larger diameter or thicker-walled, less pliable pipe) because, among other things, those “shape memory” pipes typically are much more rigid than the tubing of that '233 patent and cannot be flattened (because it would deform the pipe beyond its “shape memory” or yield point).
Other types of pipe are commonly joined by similarly relatively complex or expensive technologies. For example, PVC pipe is presently joined using a coupling that is adhered to the adjoining pipes. Currently, the most common method of joining polyethylene gas pipe is a method that end butts, melts, and forces the adjoining pipes together to form a “couple-less” joint. This method is clever and effective but requires sophisticated costly and cumbersome equipment on the job site.